Owing to the excessive demand on signal processing and space constraint, a full digital implementation of a large adaptive antenna array at millimeter wave frequencies is very challenging. Targeted at long range high data rate point-to-point link in the 70/80 GHz bands, a novel hybrid adaptive antenna array which consists of analogue subarrays followed by a digital beamformer is presented in this paper to overcome the digital implementation difficulty. Two subarray configurations, the interleaved subarray and the side-by-side subarray, are proposed, and two Doppler resilient adaptive angle-of-arrival estimation and beamforming algorithms, the differential beam tracking (DBT) and the differential beam search (DBS), are developed. Simulation results on the DBT and DBS performance are provided using a 64 element hybrid planar array of four 4 by 4 element subarrays with the two subarray configurations, respectively. Recursive mean square error (MSE) bounds of the developed algorithms are also analyzed and compared with simulated MSEs.

/pdf/a-hybrid-adaptive-antenna-array-y16abt94g6.pdf

A hybrid adaptive antenna array

In this paper, we present a novel low-complexity technique, which improves the performance of single-antenna multi-carrier communication systems, suffering from in-phase and quadrature (I/Q)-imbalance (IQI) at the receiver. We refer to the proposed scheme as I/Q-imbalance self-interference coordination (IQSC). This technique not only mitigates the detrimental effects of IQI, but, through appropriate signal processing, also coordinates the self-interference terms produced by IQI in order to achieve second-order frequency diversity. However, these benefits come at the expense of a reduction in transmission rate. More specifically, IQSC is a simple transmit diversity scheme that improves the signal quality at the receiver by elementary signal processing operations across symmetric (mirror) pairs of subcarriers. Thereby, the proposed transmission protocol has a similar complexity as Alamouti’s space-time block coding scheme and does not require extra transmit power nor any feedback. To evaluate the performance of IQSC, we derive closed-form expressions for the resulting outage probability and symbol-error rate. Interestingly, IQSC outperforms not only existing IQI compensation schemes but also the ideal system without IQI for the same spectral efficiency and practical target error rates, while it achieves almost the same performance as ideal (i.e., IQI-free) equal-rate repetition coding. Our findings reveal that IQSC is a promising low-complexity technique for significantly increasing the reliability of low-cost devices that suffer from high levels of IQI.

I/Q-Imbalance Self-Interference Coordination

During the last decades, wireless communications have evolved from a scarce technology, used by professionals for niche applications to a rapidly advancing research field. Ever increasing proliferation of smart devices, the introduction of new emerging multimedia applications, together with an exponential rise in wireless data (multimedia) demand and usage are already creating a significant burden on existing wireless systems. Future wireless networks, with improved data rates, capacity, latency, and quality of service (QoS) requirements, are expected to be the panacea of most of the current wireless systems’ problems. Interference management is critical towards this goal, whereas transceiver design and implementation are expected to play an important role. This thesis investigates the influence of interference in wireless systems, revisits promising network- and user-side interference management solutions, as well as studies the impact of interference, caused by hardware imperfections, on the performance of the wireless link and propose countermeasures. The thesis is divided into two parts. In the first part of the thesis, different types of interference and modern interference management solutions, which are expected to be used in the future wireless networks, are reviewed. Moreover, the influence of fading and interference, due to the existence of multiple possible users operating simultaneously in the same carrier frequency, on the spectrum sensing capability of a low-complexity energy detector (ED) is investigated. Analytical tools for the performance evaluation of this problem, i.e., the false alarm and detection probabilities, are derived, while the problem of appropriately selecting the energy detection threshold and the spectrum sensing duration, in order to satisfy a specific requirement, is discussed. The results reveal the detrimental effect of interference and the importance of taking into consideration the wireless environment when evaluating the ED spectrum sensing performance and selecting the ED threshold. Finally, the offered analytical framework can be applied in cognitive radio systems, which are included in several wireless standards, and are expected to be employed in ultra-dense wireless environments. The second part of this thesis investigates the impact of transceivers radio frequency (RF) front-end imperfections on the performance of the wireless system. RF imperfections generally result to signal distortion in single-carrier communications, while, in multi-carrier communications, they additionally cause interference. In both cases, RF imperfections may cause significant degradation to the quality of the wireless link, which becomes more severe as the data rates increases. Motivated by this, after briefly illustrating the influence of different types of RF imperfections, namely in-phase and quadrature imbalance (IQI), phase noise, and amplifiers non-linearities, the analytical framework for the evaluation and quantification of the effect of IQI on wireless communications in the context of cascaded fading channels, is derived. To this end, closed-form expressions for the outage probability over N∗Nakagami-m channels for both the cases of single- and multi-carrier communications, when at least one communication node suffers from IQI, are provided. To justify the importance and practical usefulness of the analysis, the offered expressions along with several deduced corresponding special cases are employed in the context of vehicle-to-vehicle communications. This study gives critical insight for the performance degradation in wireless communications, due to RF imperfections, and indicates the need for designing proper RF imperfections compensation techniques. Next, the impact of IQI and partial successive interference suppression (SIS) in the spectrum sensing of full duplex CR systems, for both the cases of single- and multi-carrier ED, is studied. In this context, closed-form expressions are derived for the false alarm and detection probabilities, in the general case, where partial SIS and joint transmitter and receiver IQI are considered. The derived expressions can be used in order to properly select the energy detection threshold that maximizes the ED spectrum sensing capabilities. Additionally, the joint influence of fading and several RF impairments on energy detection based spectrum sensing for CR systems in multi-channel environments is investigated. After assuming flat-fading Rayleigh channels and complex Gaussian transmitted signals, as well as proving that, for a given channel realization, the joint effect of RF impairments can be modeled as a complex Gaussian process, closed-form expressions for the probabilities of false alarm and detection are derived. Based on these expressions, the impact of RF impairments and fading on the spectrum sensing capability of the ED is studied. The results illustrated the degrading influence of RF imperfections on the ED spectrum sensing performance, which bring significant losses in the utilization of the spectrum. Furthermore, the impact of uncompensated IQI on orthogonal frequency division multiple access (OFDMA) systems, in which a power allocation (PA) policy is employed in order to maximize each user’s capacity, is demonstrated. To overcome, the user’s capacity loss, due to IQI, a novel, low-complexity PA strategy is presented, which, by taking into account the levels of IQI of the served users, notably enhance each user’s achievable capacity. Finally, a novel low-complexity scheme, which improves the performance of single-antenna multi-carrier communication systems, suffering from IQI at the receiver, is proposed. The proposed scheme, which we refer to as I/Q imbalance self-interference coordination (IQSC), not only mitigate the detrimental effect of IQI, but, through appropriate signal processing, also coordinates the self-interference terms produced by IQI, in order to achieve second-order frequency diversity. In order to evaluate the performance of IQSC, closed-form expressions for the resulting outage probability and symbol error rate are derived. The findings reveal that IQSC is a promising low-complexity technique for significantly increasing the reliability of low-cost devices that suffer from high levels of IQI.

Interference mitigation techniques in modern wireless communication systems

Adaptive interference mitigation requires significant resources due to recursive processing. Specific to satellite systems, interference mitigation by employing adaptive beamforming at the gateway or at the satellite both have associated problems. While ground based beamforming reduces the satellite payload complexity, it results in added feeder link bandwidth requirements, higher gateway complexity and suffers from feeder link channel degradations. On the other hand, employing adaptive beamforming onboard the satellite gives more flexibility in case of variation in traffic dynamics and also for changing of beam patterns. However, these advantages come at the cost of additional complexity at the satellite. In pursuit of retaining the benefits of onboard beamforming and to reduce the complexity associated with adaptive processing, we here propose a novel semi-adaptive beamformer for a Hybrid Terrestrial-Satellite Mobile System. The proposed algorithm is a dual form of beamforming that enables adaptive and non-adaptive processing to coexist via a robust gradient based switching mechanism. We present a detailed complexity analysis of the proposed algorithm and derive bounds associated with its power requirements. In the scenarios studied, results show that the proposed algorithm consumes up to 98% less filter computing power as compared to full-adaptive case without compromising on system performance.

/pdf/semi-adaptive-beamforming-for-ofdm-based-hybrid-terrestrial-bzq9p6pd48.pdf

Semi-Adaptive Beamforming for OFDM Based Hybrid Terrestrial-Satellite Mobile System

In this paper, we present a novel low-complexity scheme, which improves the performance of single-antenna multi-carrier communication systems, suffering from in-phase and quadrature (I/Q)-imbalance (IQI) at the receiver. We refer to the proposed scheme as I/Q-imbalance self-interference coordination (IQSC). IQSC does not only mitigate the detrimental effects of IQI, but, through appropriate signal processing, also coordinates the self-interference terms produced by IQI in order to achieve second-order frequency diversity. However, these benefits come at the expense of a reduction in transmission rate. More specifically, IQSC is a simple transmit diversity scheme that improves the signal quality at the receiver by elementary signal processing operations across symmetric (mirror) pairs of subcarriers. Thereby, the proposed transmission protocol has a similar complexity as Alamouti's space-time block coding scheme and does not require extra transmit power nor any feedback. To evaluate the performance of IQSC, we derive closed-form expressions for the resulting outage probability and symbol error rate. Interestingly, IQSC outperforms not only existing IQI compensation schemes but also the ideal system without IQI for the same spectral efficiency and practical target error rates, while it achieves almost the same performance as ideal (i.e., IQI-free) equal-rate repetition coding. Our findings reveal that IQSC is a promising low-complexity technique for significantly increasing the reliability of low-cost devices that suffer from high levels of IQI.

In this paper, we propose adaptive beamforming schemes for orthogonal frequency-division multiplexing (OFDM) communication systems. After applying the discrete-Fourier-transform operation to the output of each antenna, one estimates the signal-plus-interference spatial correlation matrix by selecting the samples at ldquononzero-valuedrdquo subcarriers and the interference-alone spatial correlation matrix by choosing those samples at ldquozero-valuedrdquo subcarriers (also known as virtual carriers). The beamforming weight vector is the ldquolargestrdquo generalized eigenvector of the resulting matrix pencil, wherein the solution is computed in a batch fashion. By using a limited number of samples, the proposed scheme offers competitive performance compared with that of the scheme using optimal beamforming weight vectors that are calculated for each subcarrier under small-angular-spread scenarios. The adaptive solution for calculating the beamforming weight vector is also derived. The proposed schemes are shown to be suitable in scenarios of narrow angular spread with exponential power delay profile. The simulations that are presented here employ the OFDM signal model to illustrate the efficacy of the proposed algorithms.

Chia-Shu Wang

Papers

Adaptive Antenna Arrays for Interference Cancellation in OFDM Communication Systems With Virtual Carriers